sewer gas in bathroom sink

My sink was stopped up pretty bad so I pulled the trap and ran a snake down it. Cleared right up. But now when I run the faucet, I get a horrible sewer smell for about a minute or so after running the water. There is no smell until I turn the water on so the trap must be working. Any ideas on this one?

To clarify, I get the smell for a couple of minutes when first turning on hot/cold. After that no smell. I'm not at home right now but we do have an AAV in the addition upstairs (bathroom) but not sure if it's connected to the downstairs...I'll investigate when I get home. I'm on city water. I don't get the smell at any other places in the house though. Thanks for all the suggestions.